A fluid accumulator is typically used in a hydraulic system for preventing the pressure of a hydraulic fluid, e.g., an oil, from rising excessively or falling suddenly due to temporary accumulating or discharging of oil contained in the hydraulic system. For example, the accumulator temporarily accumulates oil fed from a pump while a fluid-actuated device such as a hydraulic motor is turned off. When the device is actuated again, the accumulator discharges accumulated oil and feeds it rapidly to the device until more oil from the pump reaches the device.
The accumulator is generally classified as either a bladder type accumulator or one of a piston type. A bladder type accumulator usually comprises a cylindrical shell and a gasprecharged rubber bladder incorporated therein, which is inflated and deflated in response to the change of pressure of oil contained in the hydraulic system. A piston type accumulator comprises a cylindrical shell and a reciprocating piston therein, which changes the volume of an oil chamber and that of a gas chamber bounded thereby. The behavior of the bladder in response to oil flows into or out from an accumulator is more sensitive than that of the piston and accordingly, a bladder type accumulator is very often adopted in a hydraulic system. The piston type accumulator, however, is not provided with a gas-filled rubber bladder therein, hence problems associated with the bladder, e.g., injury and blowout thereof, will not be encountered in its use. This means that a piston type accumulator operates safely in a hydraulic system.
The gas chamber of a piston type accumulator is precharged with inert gas at a pressure, for example, of 850 lb/in.sup.2 or 59.76 kg/cm.sup.2. When the hydraulic system is pressurized to, for example, 3,000 lb/in.sup.2 or 210.93 kg/cm.sup.2, the oil flows into the oil chamber volume. The gas chamber is thereby reduced and the internal pressure thereof rises. Oil continues to flow into the accumulator until the pressure in the gas chamber rises to 3,000 lb/in.sup.2, at which time oil in volume equal to the reduction in volume of the gas chamber is accumulated in the oil chamber. Contrarily, when oil in the accumulator is discharged therefrom, the pressure in the gas chamber decreases until it reaches 850 lb/in.sup.2 and the piston is restored to an initial position thereof.
It is difficult to perfectly prevent precharged gas in the gas chamber from leaking out of the shell or leaking into the oil chamber through minute clearances between the inner surface of the shell and covers thereof or the piston. Leakage of gas makes the initial pressure of precharged gas decrease, whereby the operational functions of the accumulator and of the hydraulic system become abnormal.
In order to maintain normal operation of the hydraulic system, it is important that the initial pressure of precharged gas is sustained in the oil chamber of an accumulator. Therefore, the precharged pressure should be checked often the users. The precharged pressure in a conventional accumulator, however, cannot readily be inspected while the accumulator is connected with the hydraulic system. When being inspected, the accumulator should therefore be released from piping of the hydraulic system. This means that the operation of the hydraulic system must be stopped.
In U.S. Pat. No. 4,714,093 the bladder type accumulator enabling a user to observe the inflation and deflation of the bladder is disclosed. The bladder type accumulator comprises a containment shell, a collapsible bladder capable of internal pressurization located within the containment shell so that the hydraulic fluid occupies a space between the inner surface of the shell and the outer surface of collapsible bladder as the fluid is accumulated in the accumulator, and a bladder condition detecting device inside the bladder actuated by movement of the collapsible bladder walls. The detecting device includes, for example, a tube inside the bladder, a slider moving along the tube and a reed switch for detecting movement of the slider by means of the magnetic force of a permanent magnet carried thereby.
The detecting device described above, however, is not available to a piston type accumulator because no collapsible bladder is provided inside the containment shell thereof. Therefore, in the bladder type accumulator troubles in fluid-actuated devices or piping in the hydraulic system can not be detected by reliance on an abnormal behavior of the piston. For instance, trouble may not be found until the device works beyond a desired range or a large quantity of oil has leaked out. The trouble, if not found at an early stage, develops further, and much time and labor will be required for repairing the system. To avoid these problems, a piston type accumulator is proposed so as to indicated the position of a piston inside the shell, connected with the hydraulic system.